1. The Field of the Invention
This invention relates to processes for the direct catalytic production of aqueous solutions of hydrogen peroxide from hydrogen and oxygen. In particular, embodiments of the present invention relate to processes that increase catalytic activity using a relatively small amount of an additive.
2. The Related Technology
Recent demand for hydrogen peroxide product has been growing significantly both globally and in North America. This growth in demand is due primarily to hydrogen peroxide's effectiveness in chemical processes and its environmental advantages. When used in a chemical process, hydrogen peroxide decomposes into oxygen and water, which are environmental friendly byproducts.
These advantages have led many to develop new processes that use hydrogen peroxide and/or to replace certain reagents with hydrogen peroxide. For example, hydrogen peroxide is an effective replacement for chlorine in pulp and paper bleaching, water treatment, and other environmental processes. The use of hydrogen peroxide in these processes has helped meet the increased demand for products from these processes, while providing a safer, more cost effective, and more environmentally friendly manufacturing process.
Currently most commercial hydrogen peroxide is produced at hydrogen peroxide production facilities and then shipped to manufacturers who use the hydrogen peroxide in their processes. Commercially produced hydrogen peroxide is typically made using an indirect anthraquinone process. The indirect process avoids handling hydrogen gas at elevated temperatures and pressures, which can create serious safety concerns.
Nevertheless, the anthraquinone process has its own safety issues and is known to have high capital and operating costs. These safety concerns and high capital costs economically prohibit producing the hydrogen peroxide on-site at the end users facility. Consequently, the hydrogen peroxide is produced in one location and then shipped. Shipping hydrogen peroxide creates additional safety problems since concentrated hydrogen peroxide can be explosive if it violently decomposes.
Many attempts have been made to produce hydrogen peroxide directly from hydrogen and oxygen-containing feedstreams. Direct synthesis of hydrogen peroxide can potentially reduce production cost and avoid the use of toxic feedstock and working solutions (e.g., anthraquinone). Known processes for directly producing hydrogen peroxide require a catalyst and feedstreams of hydrogen and oxygen. Hydrogen and oxygen in the presence of the catalyst forms hydrogen peroxide. This process is very advantageous because it uses environmentally friendly reagents (hydrogen and oxygen) and generates no waste. Furthermore, the simplicity of the direct process makes the direct process appear to be very cost effective.
Although direct catalytic synthesis of hydrogen peroxide has attracted much attention, none of the existing processes have proved to be commercially feasible. These processes typically fail because they either require hazardous operating conditions or have low reaction rates and poor product selectivity.
Before the early 1990s most developmental hydrogen peroxide direct synthesis processes used hydrogen feed gas above 10% hydrogen in air or oxygen. These concentrations are well within the flammability limits for H2 and O2 mixtures. Since air can supply the oxygen for the combustion of H2, using feedstreams of H2 within the flammability limits is extremely dangerous.
Due to safety concerns, the recent approach has been to utilize feedstreams having hydrogen concentration below about 5 vol. %. Feedstreams below about 5% are typically not explosive. However, at such low hydrogen concentration the production rates drop to unacceptably low rates.
To achieve higher rates of production, existing processes have used a supported noble metal catalyst. The noble metal is dispersed on a support, such as carbon, to enhance catalytic activity. However, the dispersion methods used have typically not controlled for selectivity of hydrogen peroxide. Consequently, these processes produce insufficient amounts of hydrogen peroxide.
While it is known that reaction media consisting of organic solvents in significant quantity can enhance the rate of hydrogen peroxide synthesis, albeit at significant risk, the reason for that enhancement is not factually known. One assumption is that the improvement is derived from an increase in the solubility of the reaction gases, especially hydrogen, in the reaction mixture. The greater solubility theoretically allows a greater concentration of dissolved reactants to reach the catalyst surface, thereby increasing the reaction rate. Consequently, the prior art teaches that the efficacious role of organic solvents in hydrogen peroxide production is tied to the use of substantial quantities of organic solvent in the reaction mixture.
However, in many cases, it is desirable to directly produce a hydrogen peroxide product that is nearly free of organic solvents. Many end users of hydrogen peroxide need product that is essentially free from organic solvents to properly perform their manufacturing processes. Thus large quantities of organic solvent must be separated out before the hydrogen peroxide can be used. This separation step is quite costly. This cost creates a conflict between the artisan's desires to enjoy the rate benefits of a substantial amount of an organic solvent in the direct hydrogen peroxide process while at the same time producing an aqueous hydrogen peroxide product without requiring downstream separation.
Therefore what is needed is a process for increasing direct catalytic hydrogen peroxide production while avoiding the avoiding the costs associated with separating out large quantities of organic solvent.